1. Field of the Invention
The present invention relates to a magnetic recording medium, and more specifically to a magnetic recording medium excellent in electromagnetic conversion characteristics and durability. The present invention also relates to a process for producing the magnetic recording medium.
2. Disclosure of the Related Art
Some magnetic recording media comprising a lower non-magnetic layer and an upper magnetic layer in this order formed on a non-magnetic support are disclosed in the prior art.
For example, Japanese Examined Patent Publication No. 1-30221(1989) discloses, xe2x80x9ca magnetic recording medium formed by forming a non-magnetic underlayer on a substrate, and then forming a magnetic layer, characterized in that the underlayer is formed by using a radiation-curing coating material which contains at least two kinds of compounds selected from (A), (B) and (c) described below:
(A) radiation-curing compound having two or more unsaturated double bonds and a molecular weight of 5000 or higher, preferably 8000 or higher,
(B) radiation-curing compound having one or more unsaturated double bond and a molecular weight of 400 or higher and lower than 5000, preferably 6000 to 3000,
(C) radiation-curing compound having one or more unsaturated double bond and a molecular weight of lower than 400;
and by irradiating with radiation to the coated substrate.xe2x80x9d However, since there has hardly been referred to a composition of the magnetic layer in the Publication, it is difficult to obtain the magnetic recording medium having satisfactory coating layer properties.
Japanese Patent Publication (B2) No. 2566085(1996) discloses, xe2x80x9ca magnetic recording medium comprising a non-magnetic layer based on a non-magnetic powder and a binder resin and a magnetic layer based on a ferromagnetic powder and a binder resin in this order formed on a non-magnetic substrate, characterized in that the magnetic layer has a thickness of 1.0 xcexcm or less and contains an abrasive material which has a Mohs hardness of 6 or higher and a larger average particle size than the thickness of the magnetic layer.xe2x80x9d And the same Publication discloses xe2x80x9ca process for producing a magnetic recording medium, characterized by preparing respectively a coating liquid for a lower non-magnetic layer including a non-magnetic powder dispersed into a binder, and a coating liquid for a magnetic layer including a ferromagnetic powder and an abrasive material having a Mohs hardness of 6 or higher and a larger average particle size than a dried thickness of the magnetic layer dispersed into a binder; coating the coating liquid for the non-magnetic layer onto the non-magnetic substrate to obtain a non-magnetic layer; and coating the coating liquid for the magnetic layer simultaneously with or successively to coating of the lower non-magnetic layer while the lower non-magnetic layer obtained is in a wet state so that a dried thickness of the magnetic layer can be 1 xcexcm or less.xe2x80x9d However, in this producing method, because the upper magnetic layer is coated while the lower non-magnetic layer is in a wet state, an interface between the upper magnetic layer and the lower non-magnetic layer becomes non-uniform and this results in output variations. Also, as the particle size of the abrasive is larger than the thickness of the magnetic layer, problems in running durability arise.
Japanese Patent Publication (B2) No.2922771(1999) discloses a magnetic recording medium having at least two coating layers on one surface of a non-magnetic support, wherein the uppermost of the coating layers is a magnetic layer having a thickness of 0.2 to 0.5 xcexcm and the magnetic layer of the uppermost layer includes Al2O3 particles containing an inorganic substance of an average particle size of 0.1 to 0.3 xcexcm. And, the same Publication discloses forming the magnetic layer by wet-on-wet coating method. According to the examples of the same Publication, in both the upper magnetic layer and a lower non-magnetic layer, a thermoplastic resin is used as a binder and the magnetic layer having a thickness of 0.4 xcexcm is formed by wet-on-wet coating method.
However, the same Publication does not particularly consider a case of a thinner thickness of the upper magnetic layer such as 0.30 xcexcm or less, and if such an upper magnetic layer having a thinner thickness is formed by wet-on-wet coating method, an interface between the upper magnetic layer and the lower non-magnetic layer becomes non-uniform and this results in output variations.
In recent years, a magnetic layer has shown a marked trend toward a thinner layer, a higher filling and a higher durability from a demand for magnetic recording media that have an increased recording density.
Accordingly, it is an object of the present invention to provide a magnetic recording medium which solves problems of the prior art, has a thinned magnetic layer, has excellent electromagnetic conversion characteristics and has an excellent durability. It is a further object of the present invention to provide a process for producing the magnetic recording medium.
The present inventors worked assiduously and has found that even a thickness of an upper magnetic layer is as thin as 0.30 xcexcm or less, a magnetic recording medium which has excellent electromagnetic conversion characteristics and an excellent durability can be obtained by using a radiation curing type binder resin as binder of a lower non-magnetic layer and by using an abrasive having a Mohs hardness of 6 or higher and a smaller average particle size than the thickness of the upper magnetic layer, and has completed the present invention.
That is, the present invention relates to a magnetic recording medium comprising a lower non-magnetic layer containing at least a carbon black and a radiation curing type binder resin on a non-magnetic support and an upper magnetic layer having a thickness of 0.30 xcexcm or less on the lower non-magnetic layer, wherein the upper magnetic layer contains at least a ferromagnetic powder, a binder resin, and an abrasive having a Mohs hardness of 6 or higher and a smaller average particle size than the thickness of the upper magnetic layer.
The thickness of the upper magnetic layer is generally 0.05 to 0.30 xcexcm. The average particle size of the abrasive is smaller than the thickness of the upper magnetic layer, and is generally 0.01 to 0.2 xcexcm.
A centerline average roughness (Ra) of the upper magnetic layer surface is preferably 1.0 nmxe2x89xa6Raxe2x89xa68.0 nm.
The abrasive preferably contains two or more kinds of abrasives which have different average particle sizes to each other.
Also, the present invention relates to a process for producing a magnetic recording medium which comprises:
preparing respectively a lower non-magnetic layer coating material including at least a carbon black dispersed into a radiation curing type binder resin, and an upper magnetic layer coating material including at least a ferromagnetic powder, and an abrasive having a Mohs hardness of 6 or higher and a smaller average particle size than a thickness of an upper magnetic layer dispersed into a binder resin,
applying the lower non-magnetic layer coating material onto a non-magnetic support, drying the coating material, and carrying out smoothing treatment of and irradiating with radiation to resulting layer to form a lower non-magnetic layer, and then
applying the upper magnetic layer coating material onto the lower non-magnetic layer, drying the coating material, and carrying out smoothing treatment of resulting layer to form an upper magnetic layer.
Furthermore, the present invention relates to the magnetic recording medium produced by said process.
According to the present invention, a magnetic recording medium having a thinned magnetic layer and being excellent in electromagnetic converting characteristics and in durability is provided.